The hypersensitive response (HR) of higher plants is characterized by the rapid, localized death of plant cells at the site of pathogen invasion. HR occurs during incompatible pathogen/host interactions, such as when a microorganism that normally causes a disease in its host plant infects a non-host plant. The response is associated with resistance against a variety of pathogens, including nematodes, fungi, viruses, and bacteria. For a review of the hypersensitive response, see Dixon et al., Annu Rev Phytopathol 32:479 (1994) and Godiard et al., Curr Opin Genet Dev 4:662 (1994).
The ability of phytopathogenic bacteria to cause HR in resistant or non-host plants is controlled by a cluster of highly conserved bacterial genes named hypersensitive response and pathogenicity (hrp) genes. Most hrp genes are involved in forming a protein secretion apparatus for harpins, heat-stable and proteinaceous proteins which elicit HR when infiltrated into the leaf intercellular spaces of non-host plants. It is known that, when added to a plant cell culture, harpins induce the exchange of H.sup.+ and K.sup.+ across the plasmalemma to generate active oxygen species (Baker et al., Plant Physiol 102:1341 [1993]).